The present invention resides generally in the field of the preparation and use of 4-substituted pyridine compounds, and in particular to novel forms of supernucleophilic 4-substituted pyridine catalysts, and nucleophilic substitution processes useful for preparing such catalysts and other 4-substituted pyridines.
As further background, it is well known that many pyridines carrying an amino (desirably tertiary amino) group at the 4-position possess supernucleophilic properties making them highly advantageous for use as catalysts in acylation and other reactions. For example, the compound 4-N,N-dimethylaminopyridine (DMAP) is used on a large scale worldwide for acylation and other reactions in the pharmaceutical and agricultural industries. Historically, the preparation of 4-substituted pyridines such as DMAP has presented several challenges.
For example, tremendous research efforts worldwide have been made to discover effective means for transforming one group at the 4-position of the pyridine ring for another. Early on, researchers were hopeful that direct exposure of the free pyridine base to appropriate reagents would result in the effective modification of the 4-position. It has turned out, however, that most modifications of interest at the 4-position occur only at the cost of extreme conditions. For instance, 2-bromopyridine can be converted to 2-aminopyridine by reaction with ammonium hydroxide, but only at high temperatures of 200° C. and under pressure. Den Hertog et al., Rec. Trav. Chim., 51, 381 (1932). Similarly, dimethylamine reacts with 4-chloropyridine only under pressure and at a temperature of 150° C. (L. Pentimalli, Gass. Chem. Ital., 94, 902 (1964)), a process unsuitable for commercial scale. Likewise unsuitable for commercial scale is the reaction of sodium or potassium amide and metal methylanilides in etheral solvents or liquid ammonia, as described in Hauser, J. Org. Chem., 15, 310 (1949). N-pyridyl-4-pyridinium chloride hydrochloride or 4-phenoxypyridine has been reacted with nucleophiles to displace at the 4-position (D. Jerchel et al., Chem. Ber., 91, 1266 (1958)). However, these starting pyridine materials are far removed from commerce and thus such processes would be problematic if contemplated on a large scale.
In light of the difficulties of 4-substitution directly on the free base pyridine, a number of processes have been developed in which the 4-position (or 2-position) of the pyridine ring is activated toward nucleophilic substitution by a modification of the ring nitrogen of the pyridine. Such processes are generally known as activation-substitution-deactivation processes, and to date have involved either the N-oxidation or quaternization of the pyridine substrate, both of which are known to activate the 2- and 4-ring positions toward nucleophilic attack and expulsion of leaving groups at these positions. N-oxidation as a means to activate the 2- and 4-ring positions of pyridine has been relatively less studied than quaternization. This may be due to the fact that the level of activation imparted by N-oxidation is lower than that of quaternization. In the latter field, it is known that 4-substituted-pyridines such as 4-cyanopyridine can be quaternized with an alkyl iodide (e.g. methyl iodide) and reacted with ammonia to form a corresponding 4-aminopyridine. Metzger et al., J. Org. Chem., 41 (15), 2621 (1978). The dequaternization of such alkyl quats, however, is problematic, as only relatively exotic reagents such as triphenylphosphene/dimethylformamide (Aumann et al., J. Chem. Soc. Chem. Commun., 32, (1973)), triphenylphosphene/acetonitrile (Kutney et al., Synth. Commun., 5 (2), 119 (1975)) and diazabicyclononane/dimethylformamide or thiourea (Ho, Synth. Commun., 3, 99 (1973)) having been reported, with each of these processes inviting significant difficulty on an industrial scale.
More recently, research efforts have yielded quaternary-activated 4-substitution processes which can be practiced with greater advantage on a commercial scale. For example, U.S. Pat. No. 4,158,093 to Bailey et al. describes a route in which a 4-substituted pyridine base is quaternized with 2- or 4-vinylpyridine in the presence of a strong acid to yield a pyridylethyl quaternary salt. This activated quat form can then be subjected to nucleophilic substitution at the 4-position, and subsequently dequaternized in the presence of caustic.
U.S. Pat. Nos. 4,672,121 and 4,772,713 both to Nummy describe processes in which the 4-substituted pyridine base is reacted with acrylamide or an alkylacrylamide as the quaternizing reagent, and the resulting carbamoyl quat or a derivative therefrom is subject to nucleophilic displacement at the 4-position, again followed by dequaternization. In these '121 and '713 patents, the quaternization is conducted in the presence of a strong acid, and the substitution and dequaternization are conducted in the presence of a strong base such as alkali metal hydroxides or carbonates, or strong amidine bases.
The above-described research efforts have culminated in the past decade-and-a-half in the successful commercialization and worldwide use of the supernucleophilic catalyst, DMAP, and have opened the door to routes to similar useful 4-substituted pyridine compounds. However, needs remain for novel and improved 4-substitution processes for pyridines, and improved product forms. Desirable processes would entail the use of readily-available starting materials and reagents while providing high purity products and minimizing and/or simplifying purification steps. Improved processes would also minimize reagent use and the need to recycle materials or handle or dispose hazardous wastes. As well, new product forms, especially of supernucleophilic 4-substituted pyridine catalysts, would avoid or reduce difficulties which have been encountered in the handling of crystalline or flaked catalyst forms which have been available to date. The present invention provides several embodiments, each of which addresses one or more of these needs.